1. FIELD OF THE INVENTION
The invention relates in general to the field of metrology. It relates specifically to a process for the sampling of an electric signal varying over time, an apparatus for the implementation of this process and an application of the process.
2. DISCUSSION OF BACKGROUND
In fiber optic metrology, it has long since been known to investigate the attenuation characteristic of glass fibers with the aid of so-called optical time domain reflectrometry (OTDR) and to determine the variation in attenuation along the fiber (see for example S. D. Personick in "The Bell Systems Technical Journal", Vol. 56, No. 3, March 1977, pp. 355-366).
In the case of such OTDR measurements, short light pulses are launched into the glass fiber to be investigated. The elastic and unelastic scattering (Rayleigh and Raman scattering) during passage of the light pulse through the fiber produces a backscattering signal which is coupled out at the fiber input and converted by means of a photodetector (for example an avalanche photodiode APD) into a corresponding electric signal.
Now it is also known to use OTDR measurements on glass fibers (optical waveguides) for high-sensitivity resolved temperature measurement (see for example Electronics Letters, Vol. 21, No. 13, June 1985, pp. 569, 570). This involves filtering out the very weak, temperature-dependent component of the Raman scattering from the backscattering signal and using a conventional boxcar or transient recorder averager to eliminate the noise in the time-dependent electric signal produced in the photodetector, in order that it can then be processed further.
In the case of the conventional processes of signal sampling and averaging by a boxcar averager or transient recorder, long integration or averaging times, of the order of minutes to hours, are required for conditioning of the very noisy signals, as averaging is performed exclusively in digital mode and represents a separate operation.
The advantages and disadvantages of the known processes can be listed in an overview as follows: